A compressed air supply device is used in vehicles of all kinds, in particular, for supplying a vehicle air spring installation with compressed air. Air spring installations can comprise leveling devices for adjusting the distance between the vehicle axle and the vehicle body. An air spring installation can comprise a number of pneumatic bellows connected pneumatically to a common line (i.e., gallery), which can raise the vehicle body as they are filled to an increasing extent, and can correspondingly lower the vehicle body as the filling decreases. As the distance between the vehicle axle and the vehicle body or ground clearance increases, spring travels become longer, making it possible to cope with even relatively large irregularities in the ground without making contact the vehicle body. Such systems are increasingly being used in all-terrain vehicles and sport utility vehicles (SUV). Particularly with SUVs having powerful engines, it is desirable to provide the vehicle, on the one hand, with a relatively small ground clearance for high speeds on the road and, on the other hand, with a relatively large ground clearance for off-road use. It is furthermore desirable to implement a change in the ground clearance as quickly as possible, which increases the demand for rapidity, flexibility, and reliability of a compressed air supply installation.
In order to ensure long-term operation of the compressed air supply device, a main pneumatic line has an air drier for drying the compressed air. This avoids accumulation of moisture in the pneumatic system. At relatively low temperatures, moisture can lead to the formation of crystals that damage valves, and can also lead to unwanted faults in the compressed air supply device and in the pneumatic installation. An air drier has a desiccant, generally in the form of a granular fill, through which the compressed air can flow, allowing the granular fill to take up moisture contained in the compressed air by adsorption. If appropriate, an air drier can be designed as a regenerative air drier. This can be accomplished by arranging for the dried compressed air from the pneumatic installation, in particular an air spring installation, to flow through the granular fill in a countercurrent or co-current pattern relative to the filling direction during each venting cycle. Regeneration of the air drier is made possible essentially by a pressure swing at the air drier, with a pressure prevailing during regeneration generally being lower in comparison with adsorption in order to enable moisture to be released from the granules. For this purpose, the vent valve arrangement can be opened, with the regeneration capacity of the air drier generally being dependent on the pressure conditions and the pressure swing in the compressed air supply device. For a “pressure swing adsorption process” of this kind, too, it is desirable to make a compressed air supply device flexible and reliable. In particular, the aim is to allow relatively quick venting, while making available an air pressure sufficiently low for regeneration of the air drier.
DE 35 42 974 A1 describes a leveling device for vehicles that is provided with air filters and by means of which a predetermined distance between the vehicle cell and the vehicle axle can be adjusted in accordance with the vehicle loading, by filling or emptying the air springs. The device has a safety valve that can be controlled by means of the pressure in the air springs. In an installation of this kind, the air drier can be regenerated via a restrictor and a nonreturn valve that opens counter to a filling direction.
DE 199 11 933 B4 describes a compressed air supply device having an air drier with a first compressed air supply line, wherein the compressed air is passed through a desiccant in the air drier, and with a second compressed air supply line, through which compressed air can flow without having to pass through the desiccant.
Many different approaches to the design of a first pneumatic connection between a compressed air feed and a pneumatic installation are known. These allow for the basic functions of a compressed air supply device when supplying air to the pneumatic installation and releasing air from the pneumatic installation. With respect to the abovementioned requirement for relatively quick venting with an air pressure that is nevertheless sufficiently low for regeneration of the air drier, however, these approaches are still in need of improvement.
DE 102 23 405 B4 describes an air spring system on a motor vehicle with a compressed air supply device having a delivery line for connecting a compressor to the air springs and a vent line, via which the delivery line can be connected to atmosphere while being capable of being shut off by means of a switching valve. A segment of the delivery line, in which a restrictor is arranged in parallel with a nonreturn valve and a further switching valve, the segment being designed as a pneumatic parallel circuit, is provided between a drier and level control valves in a gallery of the air spring installation. The switching valve in the vent line and the switching valve in the segment of the delivery line are connected to the same output stage of a control unit by electric control lines.
DE 101 21 582 C2 describes an air supply unit for an air suspension installation, in which a vent valve in a vent line, an air spring valve in the gallery of the air suspension installation, and an air control valve are provided. All three valves are connected to an electronic control unit. The air control valve is connected in parallel with a nonreturn valve in a segment of a delivery line between a drier and a spring valve, the segment being designed as a pneumatic parallel circuit, with the result that, although air can be taken into the air spring installation unhindered, it can only be released again in a controlled manner via the air control valve. To release compressed air from the air suspension installation, all three of the valves mentioned above are opened.
U.S. Pat. No. 6,098,967 describes a compressed air supply device of the type stated at the outset, in which a segment designed as a pneumatic parallel circuit and containing two branch lines connected in parallel is arranged in the main pneumatic line between the air drier and the air spring installation, wherein, in a first branch line, a nonreturn valve, through which there can be a flow for flow admission, and, in a second branch line, a nonreturn valve, through which there can be a flow for venting, are connected in series with a restrictor and a switching valve.
EP 1 216 860 B1 describes a leveling installation for a motor vehicle, having air springs and having a control unit which exercises open-loop or closed-loop control over the functions of filling and emptying in accordance with the level of the vehicle body. Among the elements connected to the control unit are a controllable directional control valve of a compressed air supply device and a controllable directional control valve arranged ahead of a reservoir. The controllable directional control valve of the compressed air supply device, which is subject only to fixed restriction in the open state, is arranged in parallel with a nonreturn valve.
The need for further improvement in the connection of an air spring installation to the compressed air supply device, the relatively complex nature of the connection and the large number of components involved in it are problematic in all of the compressed air supply devices described above. Owing to the fact that the abovementioned segment is generally designed as a parallel circuit in a main pneumatic line, the line is open, at least in a bypass line, or has to be opened against a counterpressure, e.g., that of a nonreturn valve.
FIG. 9 of EP 1 243 447 A2 illustrates a closed leveling installation having a gallery line, to which a number of bellows, each serving as a pressure chamber for an air spring, and a controllable isolating valve arrangement of a pneumatic installation are connected, the arrangement being formed by a first 2/2-way valve and a second 2/2-way valve arranged in series. The second directional control valve is arranged in the compressed air line between the first controllable directional control valve and the air springs. The gallery line is connected to the second directional control valve. The pneumatic installation can be filled and emptied from the compressed air supply device, which is connected to the first directional control valve, via the isolating valve arrangement on one side of a drier. A compressed air reservoir is connected to the compressed air supply device via a separate directional control valve and a separate pneumatic line on the other side of the drier.
EP 1 380 453 B1 discloses a similarly closed leveling installation for vehicles, by means of which a vehicle body is sprung relative to at least one vehicle axle. The pneumatic installation can be filled and emptied from the compressed air supply device on one side of a drier, the device being connected to a single 2/2-way valve. In this device, a compressed air reservoir is likewise connected to the compressed air supply device via a separate directional control valve and a separate pneumatic line on the other side of the drier. The compressed air reservoir is isolated from the compressed air supply device independently of an air spring installation, by means of a 4/4-way valve or two 2/2-way valves.
These pneumatic systems, in which the connection of the pneumatic installation to the necessarily closed compressed air supply device is relatively complex, can also be further improved.